Acoustic transmission section



1968 J. LENAHAN 3,36

ACOUSTIC TRANSMISSION SECTION Filed Sept. 15, 1966 FIG. I

PULSE EC HO INSTRUMENT l-NVENTOR JAMES L. LENAHAN ATTORNEYS United States Patent Ofi 3,362,501 Patented Jan. 9, 1968 ice 3,362,501 ACOUSTIC TRANSMISSION SECTIGN James L. Lenahan, Glenvievv, lll., assignor to Magnatiux Corporation, Chicago, 113., a corporation of Delaware Filed Sept. 15, 1966, Ser. No. 579,545 Claims. (Cl. 181.5)

This invention relates to acoustic apparatus and more particularly to acoustic apparatus incorporating a section which has optimum transmission characteristics while being strong, rugged, durable and efficient and while being simple and relatively inexpensive in construction. Although having other applications, the section is particularly advantageous for use with ultrasonic transducers Where it may be used as a quarter wave matching section for transmitting ultrasonic waves between a face of a transducer and a medium having a characteristic impedance substantially different from that of the transducer.

Acoustic transmission sections have heretofore been proposed having alternate layers of materials of different characteristic impedances to obtain an effective characteristic impedance which is intermediate the characteristic impedances of the material. Although capable of providing the desired effective characteristic impedance, such multi-layer sections as heretofore proposed have been ineffective in practice in that the repeated application of dynamic tensile loads to the sections causes a separation of the layers and a resultant serious loss in energy of the transferred acoustic waves.

This invention was evolved with the general object of overcoming the deficiencies of prior constructions and of providing a section which is strong, rugged and durable while being highly efficient and readily constructed.

According to this invention, at least one layer of one material having a certain characteristic impedance and having apertures therein is embedded in another material having a different characteristic impedance and having portions extending through the apertures to provide an integral bond which prevents separation of the assembly. Preferably, a plurality of layers having apertures therein are provided and in accordance with a specific feature of the invention, the apertures of one layer are out of alignment with apertures in another, with respect to the direction of transmission of acoustic energy.

According to another feature of the invention, the layer or layers having apertures therein are of a metallic material while the other material is non-metallic to provide high strength and to provide an effective characteristic impedance value in a range which is desirable for many applications.

The multi-layer section of this invention is particularly advantageous for transmission of energy with minimum loss, as in quarter wave matching sections, wherein the section at the frequency of operation of an apparatus in which it is incorporated, has a length approximately equal to a quarter wave length, or an odd number of quarter wave lengths.

In accordance with the further specific feature of the invention, each layer has substantially planar opposite end faces to obtain minimum dispersion of the acoustic energy, and maximum efliciency. Preferably, the layers may be formed by a chemical etching process, with the surfaces of the apertures being substantially at right angles to the opposite planar surfaces of the layers.

Another important feature of the invention relates to a method of constructing the section, wherein a layer of a glue or the like is applied to a flat surface after which a perforated metal layer is disposed on the glue and forced toward the surface to cause the glue to extend through the apertures thereof and to provide layers of glue on opposite sides of the metal layer, additional glue and additional metal layers being applied in a similar manner to obtain a section having the desired number of layers and the desired overall thickness. With this method, the section is readily constructed and is very strong, rugged and durable.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate preferred embodiments and in which:

FIGURE 1 illustrates in cross section an ultrasonic transducer having a transmission section constructed according to the principles of this invention, the transducer being shown diagrammatically as connected to a pulse echo instrument;

FIGURE 2 is a greatly enlarged view of a portion of the transducer of FIGURE 1, showing the construction of the transmission section of this invention;

FIGURE 3 is a cross sectional view taken substantially along line IIIIII of FIGURE 2; and

FiGURE 4 is a cross sectional view taken substantially along line IVIV of FIGURE 2.

Reference numeral Ill generally designates acoustic apparatus which comprises an ultrasonic transducer 11, shown connected to a pulse echo instrument 12. The transducer 11 comprises a piezoelectric crystal 13 having a front face 14; for emanating and receiving ultrasonic waves and having a back face 15 which is cemented to a backing member 16 of acoustically absorbent material.

A section 18 constructed according to the principles of this invention, cemented to the front face 14 and, at the requency of operation of the instrument 12, and at the resonant freqeuncy of the piezoelectric crystal 13, the section 18 has an effective length of one quarter wave length, while having an effective characteristic impedance such as to provide a match between the characteristic impedance of the crystal 13 and the characteristic impedance of the medium into which the ultrasonic waves are transmitted and from which echoes are received. The illustrated transducer 11 may preferably be used for transmitting and and receiving waves into and from water, or a medium having a characteristic impedance similar to that of water, but it will be understood that the invention is not limited thereto.

The construction of the section 18 is described in detail hereinafter in connection with FIGURES 2, 3 and 4, but it is here noted that the backing member 16 is disposed within a cylindrical metal housing 19 which carries a coaxial connector 20 for connection to the instrument 12. A central conductor of the coaxial connector 20 is connected to a cylindrical conductor 21 which is disposed within an insulating disk 22 in the housing 19. The conductor 21 is connected to one end of a lead 23 disposed within the backing member 16, the other end of the lead 23 being soldered or otherwise connected to a condcutive coating forming an electrode 24 on the back face 15 of the crystal 13. A conductive paint 25, such as a silver paint, may be applied to form an electrode on the front face 14 of the crystal 13.

The section 18 in the illustrated embodiment comprises a member 26 which is preferably of a plastic material and three layers 27, 28 and 29 which are preferably of a metallic material and which are embedded in the member 26 to divide the member 26 into four layers 26a, 26b, 26c and 26d. In accordance with this invention, the layer 27 has a plurality of apertures herein, designated by reference numeral 27a and similar p-luralities of apertures 28a and 2% are provided in the layers 28 and 29, while the member 26 has portions extending through such apertures to connect and to integrally bond together the layers of the member 26 on opposite sides of each of the layers 27, 28 and 29. Thus, portions of the member 26 extend through .5 the apertures 27a in the layer 27 to bond together the layers 26a and 26b of the member 26. This arrangement is highly advantageous in that it provides a very strong, rugged and durable assembly in which separation of layers will not occur even when operating at high acoustic power levels.

The apertures 27a, 28a and 29:: are preferably distributed uniformly throughout the layers and each aperture preferably has a size on the order of one wave length or less with the transverse width of the portions which separate the adjacent apertures being of the same order of magnitude as the width of the apertures. By way of example, in a section designed for operation 2.25 megacycles, the apertures may be 0.003 inch squares in a regular pattern as shown, and the transverse width of the portions of layers 27-29 between adjacent apertures may be 0.001 inch. VJith this arrangement, each of the layers 27-29 behaves acoustically as though it were a solid layer.

Although not essential in most applications, it is generally preferable that the apertures in the respective layers be randomly oriented, so that acustic energy passing through apertures in one layer will impinge on the solid portions of another. Thus, in the illustrated embodiment, the apertures 27a in the layer 27 may be aligned with solid portions of layer 23 and also with solid portions of a layer 29, and the intermediate layer 28 may be shifted so that the orientation of the square apertures therein is at an angle relative to the orientation of the apertures in the layers 27 and 29.

The number of layers, the thickness thereof, and the thicknesses of the layers of the member 26 are selected according to the application, to obtain the optimum effective characteristic impedance and the optimum effective length of the section. By way of example, and not by Way of limitation, in a section designed for operation at 2.25 megacycles to provide an effective length of one quarter wave length, the layers 27, 28 and 29 may be metallic layers having a thickness on the order of 0.00025 inch, while the layer portions 26a, 26b, 26c and 26d of the member 26 may each have a thickness on the order 0.0015 inch. Also by way of example and not by way of limitation, the member 26 may be in the form of a circular disk having a diameter approximately 0.5 inch and, likewise, the layers 27, 28 and 29 may each have a diameter of on the order 0.5 inch.

In applications such as in matching sections, where maximum efficiency of transmission of energy is desirable, it is important that the layers 2'7, 2d and 29 in planes parallel to each other and to the opposed faces on the member 26. Also, although it is possible to use an interwoven mesh construction for each of the layers 27, 28 and 29. it is highly desirable that the opposite faces of each of the layers 27, 28 and 29 be in parallel planes, as illustrated. With this feature, the dispersion of energy in a transverse direction is minimized, and high efficiency is obtained.

To insure that the opposite faces of each of the layers 27, 28 and 29 are in parallel planes, each of the layers of the layers in the illustrated embodiment is formed from a fiat sheet of material which is chemically etched to form the apertures therein. However, the layers may be formed by punching, stamping, burning or electrically puncturing flat sheets of material, or a woven screen may be flattened in a press.

It should be noted that when a section constructed in accordance with this invention is used for impedance matching purposes, it is generally desirable that the effective length be one quarter wave length, although it is possible to use other lengths, particularly lengths equal to an odd number of quarter wave lengths. In using a section having an effective length equal to an odd number of quarter wave lengths, the optimum characteristic impedance is determined by taking the square root of the product of the impedances of the media on opposite sides of 61 the section. By this invention, it is possible to obtain the optimum characteristic impedance for many applications, with a section which is extremely strong, rugged and durable.

Another important feature of the invention relates to the method of constructing the section 18. In accordance with this feature, a layer of a suitable glue is applied on a planar surface, after which the layer 27 may be disposed on the layer of glue, the layer being manipulated until the glue is forced out through the apertures in the member. Then a second layer may be installed, usually after applying additional glue, and then additional layers may be applied in a similar fashion. After the required number of layers are placed in position, a planar end face of a pressure member may be moved toward the surface on which the glue is initially applied, and sufiicient pressure may be applied to force the glue outwardly and to obtain the desired thickness of the member 26. Thereafter the glue may be allowed to set to form an integral one-piece plastic member having portions extending through the apertures in the layers, to provide the integral bond. It will be appreciated that any type of material can be used which is operative to provide an integral bond.

It is further noted that the transmission section of this invention is particularly advantageous in a pulse echo system such as illustrated, wherein bursts of ultrasonic energy at relatively high power levels are periodically transmitted, while weak echo signals must be received and transmitted to the crystal with minimum energy loss. In such systems, and particularly immersion systems or other systems using a water couplant or the like, there is a wide difference between the characteristic impedance of the transducer and that of the medium into which the waves are transmitted and from which they are received. With the section of this invention, a characteristic impedance can be obtained which is of an intermediate value, and at the same time, the section is very strong, rugged, durable and efficient.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

I claim as my invention:

1. In acoustic apparatus, a member of a first solid material having a certain characteristic of acoustic impedance, and impedance modifying means including at least one layer of a second material having a characteristic acoustic impedance different from said certain characteristic impedance, said layer being embedded in said first material and having a multiplicity of apertures therein, and said first material having portions extending through said apertures to integrally bond together the portions of said first material on opposite sides of said layer.

2. In acoustic apparatus as defined in claim 1, said impedance modifying means including a plurality of layers embedded in spaced generally parallel relation in said first material, each having a characteristic impedance different from said certain characteristic impedance and having a multiplicity of apertures therein with said first material having portions extending through the apertures in each layer to integrally bond together the portions of said first material on opposite sides of each layer.

3. In acoustic apparatus as defined in claim 2, said layers being so oriented that apertures in one layer are aligned with solid portions of an adjacent layer.

4. In acoustic apparatus as defined in claim 1, said first material having a relatively low characteristic acoustic impedance and said second material having a characteristic acoustic impedance substantially higher than that of said first material.

5. In acoustic apparatus as defined in claim 1, said member having opposite end faces generally parallel to said layer for acoustic wave transmission from one end face to the other, with said impedance modifying means and said member being operative to coact to provide an effective characteristic impedance with respect to said wave transmission which is intermediate the characteristic impedances of said first and second materials.

6. In acoustic apparatus as defined in claim 5, means for transmitting acoustic energy of a certain frequency into one of said end faces of said member, said member having an efiective length approximately equal to an odd number of quarter wave lengths at said frequency.

7. In acoustic apparatus as defined in claim 5, a transducer having an end face bonded to one of said opposite end faces of said member.

8. In acoustic apparatus as defined in claim 1, said layer of said second material having substantially planar, parallel opposed faces.

9. In acoustic apparatus as defined in claim 1, means for transmitting acoustic energy of a certain frequency into said member, said multiplicity of apertures having transverse dimensions on the order of one Wave length or less at said frequency.

10. In a method of constructing an acoustic Wave transmission section, the steps of applying a layer of glue to a planar surface, applying to said layer of glue a thin metallic foil having a multiplicity of apertures therein, forcing said thin metallic foil toward said surface, to cause the glue to flow through said apertures and to form a layer on the opposite side of said foil, and thereafter allowing the glue to set to form layers on opposite sides of said foil integrally bonded together by portions extending through said apertures.

No references cited.

BENJAMIN A. BORCHELT, Primary Examiner.

G. H. GLANZMAN, Assistant Examiner. 

1. IN ACOUSTIC APPARATUS, A MEMBER OF A FIRSTT SOLID MATERIAL HAVING A CERTAIN CHARACTERISTIC OF ACOUSTIC IMPEDANCE, AND IMPEDANCE MODIFYING MEANS INCLUDING AT LEAST ONE LAYER OF A SECOND MATERIAL HAVING A CHARACTERISTIC ACOUSTIC IMPEDANCE DIFFERENT FROM SAID CERTAIN CHARACTERISTIC IMPEDANCE, SAID LAYER BEING EMBEDDED IN SAID FIRST 